1. Field of the Invention
This invention relates to positive displacement pumps and more particularly to positive displacement pumps having rotary displacement mechanisms and axial inlets.
2. Brief Description of the Prior Art
Positive displacement pumps with rotary displacement mechanisms are well known in the art. Internal gear pumps, spur gear pumps, vane pumps and rotary piston pumps with nutating pistons are well known rotary positive displacement pumps. In all of these devices, the rotary mechanism is surrounded by a housing which, together with the rotary mechanism, creates chambers which increase and decrease in volume because of the rotation. The chambers increasing in volume serve as the inlet and the chambers decreasing in volume serve as the outlet. In an internal gear pump with a stationary housing, for example, an inlet is disposed in one half of the housing and an outlet is disposed in the other half. Fluid moves into the inlet chambers of the pump through the housing inlet opening because the chambers are increasing in volume. Fluid exits the outlet chambers through the housing outlet because the chambers are decreasing in volume.
Based on the conventional wisdom of the prior art, the opening for the inlet and outlet are open as wide as possible to reduce pressure drop across these openings as the fluid moves into and out of the inlet chambers. Of course, the inlet and outlet must be separated so that significant flow does not occur from the outlet to the inlet across the sealing surfaces on the rotating mechanism.
A problem in the operation of positive displacement pumps has been encountered when the rotational speeds are required to be high or the inlet pressures are required to be low. Such conditions often occur in aerospace applications where the pump must operate at high altitude and therefore low inlet pressures. Further, such applications generally require low weight which means that the pump must be of small size requiring higher rotation speeds in order to achieve sufficient volume flow.
Among the problems encountered as a result of high speed and low inlet pressure design conditions, are cavitation and inlet filling inefficiency. These problems are related in that low efficiency in filling the inlet can cause cavitation. One attempted solution to this problem is to move the trailing edge of the inlet and the leading edge of the outlet in the direction of rotation so that the inlet is enlarged and the outlet is reduced. This gives the fluid more time and space to enter the inlet. However, volumetric capability is reduced with this technique, and despite some improvement resulting from this so called advance of the inlet, cavitation and low inlet filling efficiency remain a problem.